No reason to complete a W-4, they shouldn't withhold tax. Won't hurt you to complete.
Could imply an employee / employer relationship.
Could be bad for them if they are trying to hide employees as contractors. Wouldn't affect you.
Go to the IRS home page and Look up Independent Contractor:
If Company can direct or control only the result of the work done, and not the means and methods of accomplishing the result, then your workers are probably independent contractors.
Three broad characteristics are used by the IRS to determine the relationship between businesses and workers - Behavioral Control, Financial Control, and the Type of Relationship. Behavioral Control covers facts that show whether the business has a right to direct or control how the work is done through instructions, training, or other means. Financial Control covers facts that show whether the business has a right to direct or control the financial and business aspects of the worker's job.
The example you gave sounds like they are paying you an hourly rate. They have the right (contract) to control the hours and amount of time they compensate an independent contractor.
If the two of you don't agree on your status form SS-8 can be completed and the IRS will determine the status.